This invention pertains to a system for supplying hydrogen to a hydrogen utilizing apparatus utilizing a combination of a metal hydride hydrogen storage and microcavity hydrogen storage.
Because from an environmental standpoint hydrogen can be cleanly used, because hydrogen has a large capacity for energy transference, and because there is a reversible supply of hydrogen in the form of water, the utilization of hydrogen as a fuel for many different systems is becoming incresingly important. Hydrogen can be used as a fuel in apparatuses which are powered by combustion engines wherein hydrogen is oxidized and the energy obtained in this oxidation process is used to power the engine with the only product of oxidation being water. Additionally hydrogen can be used as a fuel for electrical energy generation utilizing either the heat of combustion to drive conventional steam turbines or direct use of hydrogen within fuel cells.
At all temperatures except cryogenic temperatures hydrogen exists as a gas. The storage of large supplies of hydrogen as a gas presently is done by compressing the hydrogen and storing in large tanks. However, because the hydrogen is under high pressure it is necessary that these tanks to very strong which in turn necessitates very thick walls and heavy tanks. When hydrogen is stored as a liquid at cryogenic temperatures, as with hydrogen as a gas, cryogenic liquid hydrogen must also be contained in strong, heavy tanks and additionally there is an energy penalty in the liquefaction process. Aside from the weight disadvantages of hydrogen storage in tanks both as a liquid and a gas, the storage tank must be designed and constructed of suitable materials to accommodate and control the permeability and reactivity of hydrogen with most metals.
It has been proposed to store hydrogen chemically bound in a chemical carrier such as methylcyclohexane which is catalytically converted to toluene and hydrogen, the hydrogen being used as fuel and the toluene being recycled back to methylcyclohexane. Use of such a system requires two transporation networks, one for the delivery of the methylcyclohexane to a service station for dispensing to the consumer, the other for the return of the toluene to a reconversion plant to be hydrogenated into methylcyclohexane. This type of system is still in a semihypothetical state and much technology remains to be developed until each systems can hope to be functional.
A system now being field tested for utilizing hydrogen as a fuel to propel an automobile involves the use of a metal hydride as the carrier for the hydrogen fuel. Basically this system involves having a storage tank filled with a metal that reversibly forms a metal hydride. In the presence of hydrogen and the withdrawal of heat, the metal absorbs the hydrogen forming a metal hydride. Upon the application of heat the hydride disassociates into the metal and hydrogen allowing the hydrogen to be utilized as fuel. The heat to disassociate the metal hydride is obtained from the hot exhaust gases from the engine. Currently two metal hydride systems are being studied for use in automobiles. One system is based upon a hydride of an iron titanium alloy and the second system is based upon hydrides of magnesium alloys.
The disadvantage of a total metal hydride system is that the system is both heavy and expensive. The weight problem becomes critical in mobile applications such as automobiles, buses, etc., wherein transportation of the added weight reduces the fuel economy of the vehicle. In stationary system such as systems utilizing hydrogen in the generation of electricity, weight of the system is not the critical factor however, in these systems wherein large quantities of metal hydrides will be required the economics of the system become critical.
An additional factor to be considered in mobile systems such as automobile usage of metal hydride systems is the refueling (i.e. regenerating the metal hydride) of the on board metal hydride vehicle storage tank. During refueling the vehicle storage tank would have to be coupled to a unit which withdrew heat from the storage tank allowing for the regeneration of the metal hydride. This would require at complex hookup of hydrogen supply line and cooling line. Compared to the typically five minute stop now necessary to obtain a supply of gasoline, the regeneration of the metal hydride could require a prolonged fuel stop.